1. Field of the Invention
This invention relates to a catadioptric lens barrel which can be attached to an optical projection unit, such as a projection exposure apparatus used to manufacture semiconductor wafers. More particularly, the present invention relates to a catadioptric lens barrel structure including its support structure, wherein the lens barrel structure comprises a plurality of lens barrels aligned in parallel and at least one transverse lens barrel connecting the plurality of lens barrels.
2. Description of the Related Art
A lens barrel is a tubular mechanical structure containing a series of lenses which are aligned to transfer an image from one end of the lens barrel to the other. The lens barrel may include a sturdy threaded body and durable metal retaining rings, often referred to as sub-barrels, to carry a series of lens holders, commonly referred to as cells. Another type of lens barrel is known as a catadioptric lens barrel. A catadioptric lens barrel contains not only refractive elements, such as lenses, but also reflective elements, such as mirrors.
Both optical barrel types are commonly used in optical systems. For example, optical projection units having a lens barrel may be used in a projection exposure apparatus for production of semiconductor wafers. The projection exposure apparatus includes a reticle positioned near an entrance end of the lens barrel and a semiconductor wafer positioned near an exit end. The reticle defines a pattern of signal paths to be etched on a semiconductor wafer. The projection exposure apparatus shines light through the reticle, and transfers the pattern through the lens barrel onto a predetermined area on the semiconductor wafer. The semiconductor wafer is a silicon wafer with a photo-sensitive coating for making semiconductor devices, such as microprocessor or memory chips.
A catadioptric lens barrel system using two or more lens barrels, arranged in parallel, offers benefits not found in conventional single barrel systems. The parallelism of the lens barrels reduces the overall height of the lens barrel system, but increases the overall width. Reduced height is important for many reasons, such as for ease of transport. This type of catadioptric lens barrel design, however, poses several problems. First, keeping proper alignment of a plurality of lens barrels can be difficult. Second, the focus depth of the projection system changes as temperature changes. Coefficients of thermal expansion of the lens barrels, including their internal components, and the indices of refraction of the lens materials vary with temperature. Errors resulting from these factors collectively alter the focusing properties of the optical system. Third, the catadioptric lens barrel structure tends to be more sensitive to movement or vibration than conventional single barrel designs.
In light of the foregoing, there is a need for a catadioptric lens barrel structure that holds the lens barrels stabily and accurately, and minimizes distortion errors due to temperature changes, vibrational, or other external forces, so that the resulting image is accurately focused as it is transferred and reduced from the reticle to the wafer.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, the invention is directed to a lens barrel structure. The lens barrel structure may be used, for example, in a projection exposure apparatus in semiconductor manufacturing to reduce a pattern on a reticle through the projection exposure apparatus to a semiconductor wafer. The lens barrel structure comprises a plurality of lens barrels and a support structure. Each lens barrel has an optical axis running along a length of the lens barrel. At least one of the lens barrels comprises a plurality of sub-barrels which are serially oriented along the corresponding optical axis. Each sub-barrel is physically separated from the other sub-barrels in the series. This physical separation isolates each sub-barrel from thermal and vibrational effects, or other external forces propagated from the other sub-barrels in the series.
The present invention is also directed to a lithography system such as for use in a semiconductor manufacturing including the lens barrel structure as described above.
The present invention is further directed to a method for making a lens barrel structure. The method comprises the step of providing a plurality of lens barrels. Each lens barrel has an optical axis running along a length of the lens barrel. At least one of the lens barrels comprises physical separate, serially oriented sub-barrels. Each sub-barrel has an optical sub-axis. The method also comprises the steps of aligning the optical sub-axis of each sub-barrel in a series with the optical axis of the corresponding lens barrel, and supporting each lens barrel. A support structure for supporting the lens barrels is made of a material having a low coefficient of thermal expansion, which further reduces the effects of environmental changes on the lens barrel structure.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The objects and advantages may be obtained by means of the combinations set forth in the attached claims.